Today's rendering, whether on an electronic display such as a computer monitor coupled to a personal computer, or a television screen, or on paper display such as a paper printed using a printer coupled to a personal computer, uses “blind” or structure indifferent rules for the color choices and blends. For example, in FIG. 1, the color of the intersections of the red and blue objects does not depend on the structure of the objects. The “X” on the left of the figure is a purple color at the intersection, as are the intersections of the gridlines in the middle, as is the intersection of the circles on the right. The purple is a blend of the overlapping colors of the objects and does not depend on the fact that one object forms an “X”, another forms a grid, and another forms two overlapping circles.
CLEARTYPE® technology developed by MICROSOFT® Corporation of Redmond, Wash. represents the present state of the art in on screen font rendering. CLEARTYPE® uses a technique called sub-pixel rendering, which is generally understood to improve luminance detail and therefore produce better resolution, but which can suffer from poor chrominance detail.
Without subpixel rendering, the software in a computer treats the computer's electronic display as an array of indivisible pixels, each of which has an intensity and color that are determined by the blending of three primary colors: red, green, and blue. However, actual electronic display hardware usually implements each pixel as a group of three adjacent, independent subpixels, each of which displays a different primary color.
If the computer controlling the display knows the exact position and color of all the subpixels on the screen, it can take advantage of this aspect of the electronic display hardware to improve the apparent sharpness of the images on the screen in certain situations. If each pixel on the display actually contains three subpixels of red, green, and, blue in that fixed order, then things on the screen that are smaller than one full pixel in size can be rendered by lighting only one or two of the subpixels.
For example, if a diagonal line with a width smaller than a full pixel must be rendered, then this can be done by lighting only the subpixels with centers that belong to the line. If the line passes through the leftmost portion of the pixel, only the red subpixel is lit; if it passes through the rightmost portion of the pixel, only the blue subpixel is lit. This effectively triples the sharpness of the image at normal viewing distances; but the drawback is that the line thus drawn will show color fringes upon very close examination: at some points it might look green, at other points it might look red or blue. CLEARTYPE® and other subpixel rendering technologies do not choose a particular subpixel because the color of that subpixel is desired, but rather because of the location of the subpixel. If it so happened that the order of the subpixel colors were reversed, e.g. blue-green-red instead of red-green-blue, then subpixel rendering technology that formerly chose to illuminate only a red subpixel would now choose to render only the blue subpixel.
CLEARTYPE® uses the above method to improve the sharpness of text. When the elements of a writing system symbol are smaller than a full pixel, or when a particular line has a boundary in the middle of a pixel instead of at a pixel boundary, subpixel rendering technology lights only the appropriate subpixels of each full pixel in order to more closely follow the outlines of the symbol. Each subpixel is lighted or not lighted based on local conditions of how the symbol falls across that pixel. The overall structure of the symbol, for example the fact that it may contain an interior space, e.g. the letter “p” or “Q” (as opposed to “l” or “I”, which do not contain interior space) or the fact that it may contain two strokes that are very close together, such as “m” and “n”, is not taken into account.
While CLEARTYPE® and other subpixel rendering technologies provide improved rendering, there are certain rendering problems that remain. For example, even if graphical objects could be rendered with infinite resolution, they would still suffer from unwanted visual artifacts, such as image retention, color after-image, color vibration, flashing or pulsing phenomenon. These can be seen in structure inter-joins, intersecting lines, small interior counter-spaces, and corners. No matter what the resolution, for example even in the case of printers which can print on paper with much higher resolution than can be produced on a screen, these artifacts can still have a disturbing visual effect and interfere with optimal legibility and comfort of viewing. Moreover, current CLEARTYPE® and other sub-pixel rendering technologies are based on achieving better local luminance resolution. They do not strive for or achieve better overall object appearance based on the structural characteristics of objects.
In light of the foregoing, there is a need in the industry for a technology that goes beyond CLEARTYPE® and other subpixel rendering technologies to address the various visual artifacts and other legibility problems that occur.